Magnetic recording medium having characterized magnetic layer

ABSTRACT

The magnetic recording medium includes a magnetic layer which contains ferromagnetic hexagonal ferrite powder and a binder, in which the magnetic layer contains an abrasive and a fatty acid ester, Int (110)/Int (114) of a crystal structure of the hexagonal ferrite, determined by performing XRD analysis on the magnetic layer by using an In-Plane method, is equal to or higher than 0.5 and equal to or lower than 4.0, a squareness ratio of the magnetic recording medium in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00, FWHM before  and FWHM after  is greater than 0 nm and equal to or smaller than 7.0 nm, and a difference between spacings measured within a surface of the magnetic layer by an optical interference method before and after the heating in a vacuum is greater than 0 nm and equal to or smaller than 8.0 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese Patent Application No. 2017-140025 filed on Jul. 19, 2017. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium.

2. Description of the Related Art

Generally, either or both of the recording of information on a magnetic recording medium and the reproduction of information performed by causing a magnetic head (hereinafter, simply described as “head” as well) to contact and slide on a surface of the magnetic recording medium (a surface of a magnetic layer).

In order to continuously or intermittently repeat the reproduction of the information recorded on the magnetic recording medium, the head is caused to repeatedly slide on the surface of the magnetic layer (repeated sliding). For improving the reliability of the magnetic recording medium as a recording medium for data storage, it is desirable to inhibit the deterioration of electromagnetic conversion characteristics during the repeated sliding. This is because a magnetic recording medium in which the electromagnetic conversion characteristics thereof hardly deteriorate during the repeated sliding can keep exhibiting excellent electromagnetic conversion characteristics even though the reproduction is continuously or intermittently repeated.

Examples of causes of the deterioration of electromagnetic conversion characteristics during the repeated sliding include the occurrence of a phenomenon (referred to as “spacing loss”) in which a distance between the surface of the magnetic layer and the head increases. Examples of causes of the spacing loss include a phenomenon in which while reproduction is being repeated and the head is continuously sliding on the surface of the magnetic layer, foreign substances derived from the magnetic recording medium are attached to the head. In the related art, as a countermeasure for the head attachment occurring as above, an abrasive has been added to the magnetic layer such that the surface of the magnetic layer performs a function of removing the head attachment (for example, see JP2005-243162A).

SUMMARY OF THE INVENTION

It is preferable to add an abrasive to the magnetic layer, because then it is possible to inhibit the deterioration of the electromagnetic conversion characteristics resulting from the spacing loss that occurs due to the head attachment. Incidentally, in a case where the deterioration of the electromagnetic conversion characteristics can be suppressed to a level that is higher than the level achieved by the addition of an abrasive to the magnetic layer as in the related art, it is possible to further improve the reliability of the magnetic recording medium as a recording medium for data storage.

One aspect of the present invention is based on the above circumstances, and provides for a magnetic recording medium in which the electromagnetic conversion characteristics thereof hardly deteriorate even if a head repeatedly slides on a surface of a magnetic layer.

An aspect of the present invention is a magnetic recording medium comprising a non-magnetic support and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive and a fatty acid ester, an intensity ratio (Int (110)/Int (114)) (hereinafter, described as “XRD (X-ray diffraction) intensity ratio” as well) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, a squareness ratio of the magnetic recording medium in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00, a full width at half maximum of a spacing distribution measured within a surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum is greater than 0 nm and equal to or smaller than 7.0 nm, a full width at half maximum of a spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum is greater than 0 nm and equal to or smaller than 7.0 nm, and a difference (S_(after)−S_(before)) between a spacing S_(after), which is measured within a surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum, and a spacing S_(before), which is measured within the surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum, is greater than 0 nm and equal to or smaller than 8.0 nm.

In one aspect, the squareness ratio in a vertical direction may be equal to or higher than 0.65 and equal to or lower than 0.90.

In one aspect, the full width at half maximum of a spacing distribution measured within the surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum may be equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.

In one aspect, the full width at half maximum of the spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum may be equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.

In one aspect, the difference (S_(after)−S_(before)) may be equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.

In one aspect, the magnetic recording medium may further comprise a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer.

In one aspect, the magnetic recording medium may further comprise a back coating layer containing non-magnetic powder and a binder on a surface, which is opposite to a surface provided with the magnetic layer, of the non-magnetic support.

In one aspect, the magnetic recording medium may be a magnetic tape.

According to an aspect of the present invention, it is possible to provide a magnetic recording medium in which the electromagnetic conversion characteristics thereof hardly deteriorate even though a head is caused to repeatedly slide on a surface of a magnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing the constitution of a vibrating apparatus used in examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An aspect of the present invention relates to a magnetic recording medium including a non-magnetic support and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive, an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, a squareness ratio of the magnetic recording medium in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00, a full width at half maximum of a spacing distribution measured within a surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum is greater than 0 nm and equal to or smaller than 7.0 nm, a full width at half maximum of a spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum is greater than 0 nm and equal to or smaller than 7.0 nm, and a difference (S_(after)−S_(before)) between a spacing S_(after), which is measured within a surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum, and a spacing S_(before), which is measured within the surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum, is greater than 0 nm and equal to or smaller than 8.0 nm.

In the present invention and the present specification, “surface of the magnetic layer” refers to a surface of the magnetic recording medium on the magnetic layer side. Furthermore, in the present invention and the present specification, “ferromagnetic hexagonal ferrite powder” refers to an aggregate of a plurality of ferromagnetic hexagonal ferrite particles. The ferromagnetic hexagonal ferrite particles are ferromagnetic particles having a hexagonal ferrite crystal structure. Hereinafter, the particles constituting the ferromagnetic hexagonal ferrite powder (ferromagnetic hexagonal ferrite particles) will be described as “hexagonal ferrite particles” or simply as “particles” as well. “Aggregate” is not limited to an aspect in which the particles constituting the aggregate directly contact each other, and also includes an aspect in which a binder, an additive, or the like is interposed between the particles. The same points as described above will also be applied to various powders such as non-magnetic powder in the present invention and the present specification.

In the present invention and the present specification, unless otherwise specified, the description relating to a direction and an angle (for example, “vertical”, “orthogonal”, or “parallel”) includes a margin of error accepted in the technical field to which the present invention belongs. For example, the aforementioned margin of error means a range less than a precise angle ±10°. The margin of error is preferably within a precise angle ±5°, and more preferably within a precise angle ±3°.

Hereinafter, the Full Width at Half Maximum (FWHM) of a spacing distribution measured within a surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum will be described as “FWHM_(before) of the magnetic layer” or “FWHM_(before)” as well, and the full width at half maximum of a spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum will be described as “FWHM_(after) of the magnetic layer” or “FWHM_(after)” as well. Furthermore, a difference between a spacing S_(after), which is measured within a surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum, and a spacing S_(before), which is measured within the surface of the magnetic layer by an optical interference method before the magnetic tape is heated in a vacuum, is will be described as “difference (S_(after)−S_(before)) of the magnetic layer” or “difference S_(after)−S_(before)” as well.

Regarding the aforementioned magnetic recording medium, the inventors of the present invention made assumptions as below.

The magnetic layer of the magnetic recording medium contains an abrasive. The addition of the abrasive to the magnetic layer enables the surface of the magnetic layer to perform a function of removing the head attachment. However, it is considered that in a case where the abrasive present on the surface of the magnetic layer and/or in the vicinity of the surface of the magnetic layer fails to appropriately permeate the inside of the magnetic layer by the force applied thereto from the head when the head is sliding on the surface of the magnetic layer, the head will be scraped by contacting the abrasive protruding from the surface of the magnetic layer (head scraping). It is considered that in a case where the head scraping that occurs as above can be inhibited, it is possible to further inhibit the deterioration of the electromagnetic conversion characteristics caused by the spacing loss.

Regarding the aforementioned point, the inventors of the present invention assume that in the ferromagnetic hexagonal ferrite powder contained in the magnetic layer include particles (hereinafter, referred to as “former particles”) which exert an influence on the degree of permeation of the abrasive by supporting the abrasive pushed into the inside of the magnetic layer and particles (hereinafter, referred to as “latter particles”) which are considered not to exert such an influence or to exert such an influence to a small extent. It is considered that the latter particles are fine particles resulting from partial chipping of particles due to the dispersion treatment performed at the time of preparing a composition for forming a magnetic layer, for example. The inventors of the present invention also assume that the more the fine particles contained in the magnetic layer, the further the hardness of the magnetic layer decreases, although the reason is unclear. In a case where the hardness of the magnetic layer decreases, the surface of the magnetic layer is scraped when the head slides on the surface of the magnetic layer (magnetic layer scraping), the foreign substances occurring due to the scraping are interposed between the surface of the magnetic layer and the head, and as a result, spacing loss occurs.

The inventors of the present invention consider that in the ferromagnetic hexagonal ferrite powder present in the magnetic layer, the former particles are particles resulting in a diffraction peak in X-ray diffraction analysis using an In-Plane method, and the latter particles do not result in a diffraction peak or exert a small influence on a diffraction peak because they are fine. Therefore, the inventors of the present invention assume that based on the intensity of the diffraction peak determined by X-ray diffraction analysis performed on the magnetic layer by using the In-Plane method, the way the particles, which support the abrasive pushed into the inside of the magnetic layer and exert an influence on the degree of permeation of the abrasive, are present in the magnetic layer can be controlled, and as a result, the degree of permeation of the abrasive can be controlled. The inventors of the present invention consider that the XRD intensity ratio, which will be specifically described later, is a parameter relating to the aforementioned point.

Meanwhile, the squareness ratio in a vertical direction is a ratio of remnant magnetization to saturation magnetization measured in a direction perpendicular to the surface of the magnetic layer. The smaller the remnant magnetization, the lower the ratio. Presumably, it is difficult for the latter particles to retain magnetization because they are fine. Therefore, presumably, as the amount of the latter particles contained in the magnetic layer increases, the squareness ratio in a vertical direction tends to be reduced. Accordingly, the inventors of the present invention consider that the squareness ratio in a vertical direction can be a parameter of the amount of the fine particles (the latter particles described above) present in the magnetic layer. The inventors of the present invention consider that as the amount of such fine particles contained in the magnetic layer increases, the hardness of the magnetic layer may decrease, and accordingly, the surface of the magnetic layer may be scraped when the head slides on the surface of the magnetic layer, the foreign substances that occur due to the scraping may be interposed between the surface of the magnetic layer and the head, and hence the spacing loss strongly tends to occur.

The inventors of the present invention consider that in the aforementioned magnetic recording medium, each of the XRD intensity ratio and the squareness ratio in a vertical direction is in the aforementioned range, and this makes a contribution to the inhibition of the deterioration of the electromagnetic conversion characteristics during the repeated sliding. According to the inventors of the present invention, presumably, this is because the control of the XRD intensity ratio mainly makes it possible to inhibit the head scraping, and the control of the squareness ratio in a vertical direction mainly makes it possible to inhibit the magnetic layer scraping.

The inventors of the present invention consider that, in the aforementioned magnetic recording medium, the state where each of FWHM_(before), FWHM_(after), and the difference (S_(after)−S_(before)) of the magnetic layer is within the aforementioned range makes a contribution to the further inhibition of the deterioration of electromagnetic conversion characteristics during the repeated sliding. This point will be further described below.

FWHM_(after) and the spacing S_(after) for determining the difference (S_(after)−S_(before)) are values determined after the magnetic recording medium is heated in a vacuum. In the present invention and the present specification, the magnetic recording medium is “heated in a vacuum” by being kept for 24 hours in an environment with an atmospheric temperature of 70° C. to 90° C. under a pressure of equal to or higher than 200 Pa and equal to or lower than 0.01 MPa.

In the present invention and the present specification, the spacing measured within a surface of the magnetic layer of the magnetic recording medium by an optical interference method is a value measured by the following method.

The magnetic recording medium and a transparent plate-like member (for example, a glass plate, or the like) are stacked such that the surface of the magnetic layer of the magnetic recording medium and the transparent plate-like member face each other. In this state, the magnetic recording medium is pressed from a side opposite to the magnetic layer side by a pressing member under a pressure of 5.05×10⁴ N/m (0.5 atm). In this state, the surface of the magnetic layer of the magnetic recording medium is irradiated with light (irradiation area: 150,000 to 200,000 μm²) through the transparent plate-like member, and based on the intensity of interference light (for example, the contrast of an interference fringe image) that occurs due to an optical path difference between light reflected from the surface of the magnetic layer of the magnetic recording medium and light reflected from the surface of the transparent plate-like member on the magnetic recording medium side, a spacing (distance) between the surface of the magnetic layer of the magnetic recording medium and the surface of the transparent plate-like member on the magnetic recording medium side is determined. Herein, the radiated light is not particularly limited. In a case where the radiated is light having an emission wavelength that covers a relatively wide range of wavelength just like white light including light of a plurality of wavelengths, between the transparent plate-like member and a light-receiving portion that receives the reflected light, a member such as a dichroic filter functioning to selectively cut light having a specific wavelength or light having a wavelength other than a specific wavelength is disposed, such that light of some wavelengths or light of some wavelength bands is allowed to be incident on the light-receiving portion among the reflected lights. In a case where the radiated light is light having a single emission peak (so-called monochromatic light), the aforementioned member may not be used. The wavelength of the light allowed to be incident on the light-receiving portion can be within a range of 500 to 700 nm, for example. However, the wavelength of the light allowed to be selectively incident on the light-receiving portion is not limited to the above range. Furthermore, the transparent plate-like member may be a member that is transparent enough for transmitting the radiated light such that interference light is obtained by irradiating the magnetic recording medium with light through the member.

The aforementioned measurement can be performed using, for example, a commercial Tape Spacing Analyzer (TSA) such as Tape Spacing Analyzer manufactured by MicroPhysics, Inc. In examples, the spacing was measured using Tape Spacing Analyzer manufactured by MicroPhysics, Inc.

In the present invention and the present specification, in order to determine the full width at half maximum of a spacing distribution, the interference fringe image obtained by measuring the spacing is divided into 300,000 points, a spacing (distance between the surface of the magnetic layer of the magnetic recording medium and the surface of the transparent plate-like member on the magnetic recording medium side) is determined for each point and made into a histogram. In a case where the Gaussian distribution is fitted to the histogram, the Full Width at Half Maximum (FWHM) is determined.

The difference (S_(after)−S_(before)) refers to a value obtained by subtracting a mode before the heating in a vacuum from a mode after the heating in a vacuum in the aforementioned 300,000 points.

Regarding various values relating to the magnetic layer, the inventors of the present invention made assumptions described below in (1) and (2).

(1) Usually, within the surface of the magnetic layer, there are a portion (projection) which mainly comes into contact (so-called real contact) with a head in a case where the head slides on the surface of the magnetic layer and a portion (hereinafter, referred to as “base material portion”) which is lower than the aforementioned portion. The inventors of the present invention consider that the spacing described above is a value that becomes a parameter of the distance between the head and the base material portion in a case where the head slides on the surface of the magnetic layer. Here, the inventors consider that in a case where a lubricant contained in the magnetic layer forms a liquid film on the surface of the magnetic layer, due to the presence of the liquid film between the base material portion and the head, the spacing is narrowed by the thickness of the liquid film.

Incidentally, the lubricant is roughly classified into a fluid lubricant and a boundary lubricant in general. The fatty acid ester contained in the magnetic layer of the magnetic recording medium is said to be a component that can function as a fluid lubricant. The fluid lubricant is considered to be able to play a role of protecting the surface of the magnetic layer by forming a liquid film on the surface of the magnetic layer. The inventors of the present invention consider that the presence of the liquid film of the fatty acid ester on the surface of the magnetic layer makes a contribution to the smooth sliding (improvement of sliding) of the head on the surface of the magnetic layer. Here, the inventors consider that in a case where an excess of fatty acid ester is present on the surface of the magnetic layer, a meniscus (liquid crosslink) is formed between the surface of the magnetic layer and the head due to the fatty acid ester and causes sticking, and hence the sliding deteriorates.

Regarding the aforementioned point, the inventors of the present invention focused on the fact that the fatty acid ester is a component having a property of volatilizing by heating in a vacuum, and adopted the difference between the spacing after the heating in a vacuum (state where the liquid film of the fatty acid ester has volatilized and been removed) and the spacing before the heating in a vacuum (state where the liquid film of the fatty acid ester is present) as a parameter of the thickness of the liquid film formed of the fatty acid ester on the surface of the magnetic layer. The inventors of the present invention assume that in a case where the liquid film of the fatty acid ester is present on the surface of the magnetic layer such that the difference (S_(after)−S_(before)) becomes greater than 0 nm and equal to or smaller than 8.0 nm, the occurrence of sticking may be inhibited, and the sliding of the head on the surface of the magnetic layer may be improved. The inventors consider that in a case where the sliding can be improved, it is possible to inhibit the magnetic layer from experiencing scraping due to the damage of the surface of the magnetic layer when the head slides on the surface of the magnetic layer. The inventors of the present invention assume that the inhibition of the occurrence of magnetic layer scraping may lead to the inhibition of the deterioration of electromagnetic conversion characteristics that occurs during the repeated sliding by the occurrence of the spacing loss due to foreign substances caused by the magnetic layer scraping.

(2) A small full width at half maximum of a spacing distribution means that the variation of the value of spacing measured within each portion of the surface of the magnetic layer is small. As a result of repeating intensive examinations, the inventors of the present invention concluded that in order to improve the sliding of the head on the surface of the magnetic layer, it is effective to improve the uniformity of the height of the projection present on the surface of the magnetic layer and to improve the uniformity of the thickness of the liquid film of the fatty acid ester, such that the uniformity of the contact state between the surface of the magnetic layer and the head is improved.

Regarding this point, the variation of the value of the spacing is considered to be caused by the variation of the height of the projection on the surface of the magnetic layer and the variation of the thickness of the liquid film of the fatty acid ester. The inventors of the present invention assume that the full width at half maximum FWHM_(before) of the spacing distribution measured before the heating in a vacuum, that is, measured in a state where the liquid film of the fatty acid ester is present on the surface of the magnetic layer may increase as the variation of the height of the projection and the variation of the thickness of the liquid film of the fatty acid ester increase. The inventors assume that FWHM_(before) may be greatly affected particularly by the variation of the thickness of the liquid film of the fatty acid ester. The inventors of the present invention consider that, in contrast, the full width at half maximum FWHM_(after) of the spacing distribution measured after the heating in a vacuum, that is, measured in a state where the liquid film of the fatty acid ester has been removed from the surface of the magnetic layer increases as the variation of the height of the projection increases. In other words, the inventors of the present invention assume that the smaller the both the full widths at half maximum FWHM_(before) and FWHM_(after) of the spacing distribution, the smaller the variation of the thickness of the liquid film of the fatty acid ester and the variation of the height of the projection on the surface of the magnetic layer. Furthermore, the inventors consider that improving the uniformity of the height of the projection and the thickness of the liquid film of the fatty acid ester such that both the full widths at half maximum FWHM_(before) and FWHM_(after) of the spacing distribution become greater than 0 nm and equal to or smaller than 7.0 nm makes a contribution to the improvement of the sliding of the head on the surface of the magnetic layer. The inventors of the present invention assume that the improvement of the sliding of the head on the surface of the magnetic layer may lead to the inhibition of the deterioration of electromagnetic conversion characteristics during the repeated sliding.

The points described so far are assumptions that the inventors of the present invention made regarding the mechanism which makes it possible to inhibit the deterioration of the electromagnetic conversion characteristics in the magnetic recording medium even though the head repeatedly slides on the surface of the magnetic layer. However, the present invention is not limited to the assumption. The present specification includes the assumption of the inventors of the present invention, and the present invention is not limited to the assumption.

Hereinbelow, various values will be more specifically described.

XRD Intensity Ratio

In the magnetic recording medium, the magnetic layer contains ferromagnetic hexagonal ferrite powder. The XRD intensity ratio is determined by performing X-ray diffraction analysis on the magnetic layer containing the ferromagnetic hexagonal ferrite powder by using an In-Plane method. Hereinafter, the X-ray diffraction analysis performed using an In-Plane method will be described as “In-Plane XRD” as well. In-Plane XRD is performed by irradiating the surface of the magnetic layer with X-rays by using a thin film X-ray diffractometer under the following conditions. Magnetic recording media are roughly classified into a tape-like magnetic recording medium (magnetic tape) and a disc-like magnetic recording medium (magnetic disc). The magnetic tape is measured in a longitudinal direction, and the magnetic disc is measured in a radius direction.

-   -   Radiation source used: Cu radiation (power of 45 kV, 200 mA)     -   Scan condition: 0.05 degree/step within a range of 20 to 40         degree, 0.1 degree/min     -   Optical system used: parallel optical system     -   Measurement method: 2θχ scan (X-ray incidence angle: 0.25°)

The above conditions are values set in the thin film X-ray diffractometer. As the thin film X-ray diffractometer, known instruments can be used. As one of the thin film X-ray diffractometers, SmartLab manufactured by Rigaku Corporation can be exemplified. The sample used for In-Plane XRD analysis is not limited in terms of the size and shape, as long as it is a medium sample which is cut from a magnetic recording medium to be measured and enables the confirmation of a diffraction peak which will be described later.

Examples of the techniques of X-ray diffraction analysis include thin film X-ray diffraction and powder X-ray diffraction. By the powder X-ray diffraction, the X-ray diffraction of a powder sample is measured. In contrast, by the thin film X-ray diffraction, it is possible to measure the X-ray diffraction of a layer formed on a substrate and the like. The thin film X-ray diffraction is classified into an In-Plane method and an Out-Of-Plane method. In the Out-Of-Plane method, the X-ray incidence angle during measurement is within a range of 5.00° to 90.00°. In contrast, in the In-Plane method, the X-ray incidence angle is generally within a range of 0.20° to 0.50°. In the present invention and the present specification, the X-ray incidence angle in In-Plane XRD is set to be 0.25° as described above. In the In-Plane method, the X-ray incidence angle is smaller than in the Out-Of-Plane method, and hence the X-ray permeation depth is small. Accordingly, by the X-ray diffraction analysis (In-Plane XRD) using the In-Plane method, it is possible to analyze the X-ray diffraction of a surface layer portion of a sample to be measured. For the sample of the magnetic recording medium, the X-ray diffraction of the magnetic layer can be analyzed by In-Plane XRD. In an X-ray diffraction spectrum obtained by the aforementioned In-Plane XRD, the aforementioned XRD intensity ratio is an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure. Int is used as the abbreviation of intensity. In the X-ray diffraction spectrum obtained by In-Plane XRD (ordinate: intensity, abscissa: diffraction angle 2θχ (degree)), the diffraction peak of (114) plane is a peak detected at 2θχ that is within a range of 33 to 36 degree, and the diffraction peak of (110) plane is a peak detected at 2θχ that is within a range of 29 to 32 degree.

Among diffraction planes, (114) plane of the crystal structure of the hexagonal ferrite is positioned close to a direction of a magnetization easy axis (c-axis direction) of the particles of the ferromagnetic hexagonal ferrite powder (hexagonal ferrite particles). The (110) plane of the hexagonal ferrite crystal structure is positioned in a direction orthogonal the direction of the magnetization easy axis.

Regarding the aforementioned former particles among the hexagonal ferrite particles contained in the magnetic layer, the inventors of the present invention considered that the more the direction of the particles orthogonal to the magnetization easy axis is parallel to the surface of the magnetic layer, the more difficult it is for the abrasive to permeate the inside of the magnetic layer by being supported by the hexagonal ferrite particles. In contrast, regarding the former particles in the magnetic layer, the inventors of the present invention consider that the more the direction of the particles orthogonal to the magnetization easy axis is perpendicular to the surface of the magnetic layer, the easier it is for the abrasive to permeate the inside of the magnetic layer because it is difficult for the abrasive to be supported by the hexagonal ferrite powder. Furthermore, the inventors of the present invention assume that in the X-ray diffraction spectra determined by In-Plane XRD, in a case where the intensity ratio (Int (110)/Int (114); XRD intensity ratio) of the peak intensity Int (110) of the diffraction peak of (110) plane to the peak intensity Int (114) of the diffraction peak of (114) plane of the hexagonal ferrite crystal structure is high, it means that the magnetic layer contains a large amount of the former particles whose direction orthogonal to the direction of the magnetization easy axis is more parallel to the surface of the magnetic layer; and in a case where the XRD intensity ratio is low, it means that the magnetic layer contains a small amount of such former particles. In addition, the inventors consider that in a case where the XRD intensity ratio is equal to or lower than 4.0, it means that the former particles, that is, the particles, which support the abrasive pushed into the inside of the magnetic layer and exert an influence on the degree of the permeation of the abrasive, merely support the abrasive, and as a result, the abrasive can appropriately permeate the inside of the magnetic layer at the time when a head slides on the surface of the magnetic layer. The inventors of the present invention assume that the aforementioned mechanism may make a contribution to hinder the occurrence of the head scraping even if the head repeatedly slides on the surface of the magnetic layer. In contrast, the inventors of the present invention consider that the state in which the abrasive appropriately protrudes from the surface of the magnetic layer when the head slides on the surface of the magnetic layer may make a contribution to the reduction of the contact area (real contact) between the surface of the magnetic layer and the head. The inventors consider that the larger the real contact area, the stronger the force applied to the surface of the magnetic layer from the head when the head slides on the surface of the magnetic layer, and as a result, the surface of the magnetic layer is damaged and scraped. Regarding this point, the inventors of the present invention assume that in a case where the XRD intensity ratio is equal to or higher than 0.5, it shows that the aforementioned former particles are present in the magnetic layer in a state of being able to support the abrasive with allowing the abrasive to appropriately protrude from the surface of the magnetic layer when the head slides on the surface of the magnetic layer.

From the viewpoint of further inhibiting the deterioration of the electromagnetic conversion characteristics, the XRD intensity ratio is preferably equal to or lower than 3.5, and more preferably equal to or lower than 3.0. From the same viewpoint, the XRD intensity ratio is preferably equal to or higher than 0.7, and more preferably equal to or higher than 1.0. The XRD intensity ratio can be controlled by the treatment conditions of the alignment treatment performed in the manufacturing process of the magnetic recording medium. As the alignment treatment, it is preferable to perform a vertical alignment treatment. The vertical alignment treatment can be preferably performed by applying a magnetic field in a direction perpendicular to a surface of the wet (undried) coating layer of the composition for forming a magnetic layer. The further the alignment conditions are strengthened, the higher the XRD intensity ratio tends to be. Examples of the treatment conditions of the alignment treatment include the magnetic field intensity in the alignment treatment and the like. The treatment conditions of the alignment treatment are not particularly limited, and may be set such that an XRD intensity ratio of equal to or higher than 0.5 and equal to or lower than 4.0 can be achieved. For example, the magnetic field intensity in the vertical alignment treatment can be set to be 0.10 to 0.80 T or 0.10 to 0.60 T. As the dispersibility of the ferromagnetic hexagonal ferrite powder in the composition for forming a magnetic layer is improved, the value of the XRD intensity ratio tends to increase by the vertical alignment treatment.

Squareness Ratio in Vertical Direction

The squareness ratio in a vertical direction is a squareness ratio measured in a vertical direction of the magnetic recording medium. “Vertical direction” described regarding the squareness ratio refers to a direction orthogonal to the surface of the magnetic layer. For example, in a case where the magnetic recording medium is a tape-like magnetic recording medium, that is, a magnetic tape, the vertical direction is a direction orthogonal to a longitudinal direction of the magnetic tape. The squareness ratio in a vertical direction is measured using a vibrating sample fluxmeter. Specifically, in the present invention and the present specification, the squareness ratio in a vertical direction is a value determined by carrying out scanning in the vibrating sample fluxmeter by applying a maximum external magnetic field of 1,194 kA/m (15 kOe) as an external magnetic field to the magnetic recording medium, at a measurement temperature of 23° C.±1° C. under the condition of a scan rate of 4.8 kA/m/sec (60 Oe/sec), which is used after being corrected for a demagnetizing field. The measured squareness ratio is a value from which the magnetization of a sample probe of the vibrating sample fluxmeter is subtracted as background noise.

The squareness ratio in a vertical direction of the magnetic recording medium is equal to or higher than 0.65. The inventors of the present invention assume that the squareness ratio in a vertical direction of the magnetic recording medium can be a parameter of the amount of the aforementioned latter particles (fine particles) present in the magnetic layer that are considered to induce the reduction in the hardness of the magnetic layer. It is considered that the magnetic layer in the magnetic recording medium having a squareness ratio in a vertical direction of equal to or higher than 0.65 has high hardness because of containing a small amount of such fine particles and is hardly scraped by the sliding of the head on the surface of the magnetic layer. Presumably, because the surface of the magnetic layer is hardly scraped, it is possible to inhibit the electromagnetic conversion characteristics from deteriorating due to the occurrence of spacing loss resulting from foreign substances that occur due to the scraping of the surface of the magnetic layer. From the viewpoint of further inhibiting the deterioration of the electromagnetic conversion characteristics, the squareness ratio in a vertical direction is preferably equal to or higher than 0.68, more preferably equal to or higher than 0.70, even more preferably equal to or higher than 0.73, and still more preferably equal to or higher than 0.75. In principle, the squareness ratio is 1.00 at most. Accordingly, the squareness ratio in a vertical direction of the magnetic recording medium is equal to or lower than 1.00. The squareness ratio in a vertical direction may be equal to or lower than 0.95, 0.90, 0.87, or 0.85, for example. The larger the value of the squareness ratio in a vertical direction, the smaller the amount of the aforementioned fine latter particles in the magnetic layer. Therefore, it is considered that from the viewpoint of the hardness of the magnetic layer, the value of the squareness ratio is preferably large. Accordingly, the squareness ratio in a vertical direction may be higher than the upper limit exemplified above.

The inventors of the present invention consider that in order to obtain a squareness ratio in a vertical direction of equal to or higher than 0.65, it is preferable to inhibit fine particles from occurring due to partial chipping of particles in the step of preparing the composition for forming a magnetic layer. Specific means for inhibiting the occurrence of chipping will be described later.

Full widths at half maximum FWHM_(before) and FWHM_(after) of spacing distribution

Both the full width at half maximum FWHM_(before) of the spacing distribution before the heating in a vacuum measured in the magnetic recording medium and the full width at half maximum FWHM_(after) of the spacing distribution after the heating in a vacuum are greater than 0 nm and equal to or smaller than 7.0 nm. The inventors of the present invention assume that the full widths at half maximum within the above range may make a contribution to the inhibition of the deterioration of electromagnetic conversion characteristics during the repeated sliding. From the viewpoint of further inhibiting the deterioration of electromagnetic conversion characteristics, each of FWHM_(before) and FWHM_(after) is preferably equal to or smaller than 6.5 nm, more preferably equal to or smaller than 6.0 nm, even more preferably equal to or smaller than 5.5 nm, still more preferably equal to or smaller than 5.0 nm, and yet more preferably equal to or smaller than 4.5 nm. FWHM_(before) and FWHM_(after) can be equal to or greater than 0.5 nm, 1.0 nm, 2.0 nm, or 3.0 nm, for example. Here, it is considered that from the viewpoint of inhibiting the deterioration of electromagnetic conversion characteristics during the repeated sliding, the smaller the value, the more preferable. Therefore, each of FWHM_(before) and FWHM_(after) may be smaller than the value exemplified above.

The full width at half maximum FWHM_(before) of the spacing distribution before the heating in a vacuum can be further reduced mainly by reducing the variation of the thickness of the liquid film of the fatty acid ester. One of the examples of specific means for doing that will be described later. In contrast, the full width at half maximum FWHM_(after) of the spacing distribution after the heating in a vacuum can be further reduced by reducing the variation of the height of the projection on the surface of the magnetic layer. In order to reduce the FWHM_(after), it is preferable to control the state of presence of a powder component contained in the magnetic layer, for example, the state where a non-magnetic filler, which will be specifically described later, is present in the magnetic layer. One of the examples of specific means for doing that will be described later.

Difference (S_(after)−S_(before))

The difference (S_(after)−S_(before)) in the spacing before and after the heating in a vacuum that is measured in the magnetic recording medium is greater than 0 nm and equal to or smaller than 8.0 nm. The inventors of the present invention assume that the difference (S_(after)−S_(before)) within the above range may also make a contribution to the inhibition of the deterioration of electromagnetic conversion characteristics during the repeated sliding. From the viewpoint of further inhibiting the deterioration of electromagnetic conversion characteristics, the difference (S_(after)−S_(before)) is preferably equal to or greater than 0.1 nm, more preferably equal to or greater than 1.0 nm, even more preferably equal to or greater than 1.5 nm, still more preferably equal to or greater than 2.0 nm, yet more preferably equal to or greater than 2.5 nm, and much more preferably equal to or greater than 3.0 nm. In contrast, from the viewpoint of further inhibiting the deterioration of electromagnetic conversion characteristics during the repeated sliding, the difference (S_(after)−S_(before)) is preferably equal to or smaller than 7.5 nm, more preferably equal to or smaller than 7.0 nm, even more preferably equal to or smaller than 6.5 nm, still more preferably equal to or smaller than 6.0 nm, yet more preferably equal to or smaller than 5.5 nm, much more preferably equal to or smaller than 5.0 nm, more preferably equal to or smaller than 4.5 nm, and even more preferably equal to or smaller than 4.0 nm. The difference (S_(after)−S_(before)) can be controlled by the amount of the fatty acid ester added to the composition for forming a magnetic layer. Furthermore, for a magnetic recording medium having a non-magnetic layer between a non-magnetic support and a magnetic layer, the difference (S_(after)−S_(before)) can also be controlled by the amount of the fatty acid ester added to the composition for forming a non-magnetic layer. The non-magnetic layer can play a role of holding a lubricant and supplying the lubricant to the magnetic layer, because the fatty acid ester contained in the non-magnetic layer can move to the magnetic layer and be present within the surface of the magnetic layer.

Hereinafter, the magnetic recording medium will be more specifically described.

Magnetic Layer

Ferromagnetic Hexagonal Ferrite Powder

The magnetic layer of the magnetic recording medium contains ferromagnetic hexagonal ferrite powder as ferromagnetic powder. Regarding the ferromagnetic hexagonal ferrite powder, a magnetoplumbite type (referred to as “M type” as well), a W type, a Y type, and Z type are known as crystal structures of the hexagonal ferrite. The ferromagnetic hexagonal ferrite powder contained in the magnetic layer may take any of the above crystal structures. The crystal structures of the hexagonal ferrite contain an iron atom and a divalent metal atom as constituent atoms. The divalent metal atom is a metal atom which can become a divalent cation as an ion, and examples thereof include alkali earth metal atoms such as a barium atom, a strontium atom, and a calcium atom, a lead atom, and the like. For example, the hexagonal ferrite containing a barium atom as a divalent metal atom is barium ferrite, and the hexagonal ferrite containing a strontium atom is strontium ferrite. The hexagonal ferrite may be a mixed crystal of two or more kinds of hexagonal ferrite. As one of the mixed crystals, a mixed crystal of barium ferrite and strontium ferrite can be exemplified.

As the parameter of a particle size of the ferromagnetic hexagonal ferrite powder, activation volume can be used. “Activation volume” is the unit of magnetization inversion. The activation volume described in the present invention and the present specification is a value measured using a vibrating sample fluxmeter in an environment with an atmospheric temperature of 23° C.±1° C. by setting a magnetic field sweep rate to be 3 minutes and 30 minutes for a coercive force Hc measurement portion, and determined from the following relational expression of Hc and an activation volume V. Hc=2 Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant, Ms: saturation magnetization, k: Boltzmann constant, T: absolute temperature, V: activation volume, A: spin precession frequency, t: magnetic field inversion time]

Examples of methods for achieving the high-density recording include a method of increasing a filling rate of ferromagnetic powder in the magnetic layer by reducing the particle size of the ferromagnetic powder contained in the magnetic layer. In this respect, the activation volume of the ferromagnetic hexagonal ferrite powder is preferably equal to or less than 2,500 nm³, more preferably equal to or less than 2,300 nm³, and even more preferably equal to or less than 2,000 nm³. In contrast, from the viewpoint of the stability of magnetization, the activation volume is preferably equal to or greater than 800 nm³, more preferably equal to or greater than 1,000 nm³, and even more preferably equal to or greater than 1,200 nm³, for example.

In order to identify the shape of the particles constituting the ferromagnetic hexagonal ferrite powder, the ferromagnetic hexagonal ferrite powder is imaged using a transmission electron microscope at a 100,000× magnification, and the image is printed on photographic paper such that the total magnification thereof becomes 500,000×. In the image of the particles obtained in this way, the outlines of particles (primary particles) are traced using a digitizer so as to identify the particle shape. The primary particles refer to independent particles not being aggregated with each other. The particles are imaged using a transmission electron microscope at an acceleration voltage of 300 kV by using a direct method. For performing observation and measurement using the transmission electron microscope, for example, it is possible to use a transmission electron microscope H-9000 manufactured by Hitachi High-Technologies Corporation and image analysis software KS-400 manufactured by Carl Zeiss AG. Regarding the shape of the particles constituting the ferromagnetic hexagonal ferrite powder, “plate-like” means a shape having two plate surfaces facing each other. Among particle shapes that do not have such plate surfaces, a shape having a major axis and a minor axis different from each other is “elliptical”. The major axis is an axis (straight line) which is the longest diameter of a particle. The minor axis is a straight line which is the longest diameter of a particle in a direction orthogonal to the major axis. A shape in which the major axis and the minor axis are the same as each other, that is, a shape in which the major axis length equals the minor axis length is “spherical”. A shape in which the major axis and the minor axis cannot be identified is called “amorphous”. The imaging performed for identifying the particle shape by using a transmission electron microscope is carried out without performing an alignment treatment on the powder to be imaged. The ferromagnetic hexagonal ferrite powder used for preparing the composition for forming a magnetic layer and the ferromagnetic hexagonal ferrite powder contained in the magnetic layer may take any of the plate-like shape, the elliptical shape, the spherical shape and the amorphous shape.

The mean particle size relating to various powders described in the present invention and the present specification is an arithmetic mean of sizes determined for 500 particles randomly extracted using a particle image captured as described above. The mean particle size shown in examples which will be described later is a value obtained using a transmission electron microscope H-9000 manufactured by Hitachi High-Technologies Corporation as a transmission electron microscope and image analysis software KS-400 manufactured by Carl Zeiss AG as image analysis software.

For details of the ferromagnetic hexagonal ferrite powder, for example, paragraphs “0134” to “0136” in JP2011-216149A can also be referred to.

The content (filling rate) of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferably within a range of 50% to 90% by mass, and more preferably within a range of 60% to 90% by mass. The magnetic layer contains at least a binder, an abrasive, and a fatty acid ester as components other than the ferromagnetic hexagonal ferrite powder, and can optionally contain one or more kinds of additives. From the viewpoint of improving the recording density, the filling rate of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferably high.

Binder and Curing Agent

The magnetic layer of the magnetic recording medium contains a binder. As the binder, one or more kinds of resins are used. The resin may be a homopolymer or a copolymer. As the binder contained in the magnetic layer, a binder selected from an acryl resin obtained by copolymerizing a polyurethane resin, a polyester resin, a polyamide resin, a vinyl chloride resin, styrene, acrylonitrile, or methyl methacrylate, a cellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin, a polyvinyl alkyral resin such as polyvinyl acetal or polyvinyl butyral can be used singly, or a plurality of resins can be used by being mixed together. Among these, a polyurethane resin, an acryl resin, a cellulose resin, and a vinyl chloride resin are preferable. These resins can be used as a binder in a non-magnetic layer and/or a back coating layer which will be described later. Regarding the aforementioned binders, paragraphs “0029” to “0031” in JP2010-24113A can be referred to. The binder may be a radiation-curable resin such as an electron beam-curable resin. Regarding the radiation-curable resin, paragraphs “0044” and “0045” in JP2011-048878A can be referred to.

The average molecular weight of the resin used as a binder can be equal to or greater than 10,000 and equal to or less than 200,000 in terms of a weight-average molecular weight, for example. The weight-average molecular weight in the present invention and the present specification is a value determined by measuring a molecular weight by gel permeation chromatography (GPC) and expressing the molecular weight in terms of polystyrene. As the measurement conditions, the following conditions can be exemplified. The weight-average molecular weight shown in examples which will be described later is a value determined by measuring a molecular weight under the following measurement conditions and expressing the molecular weight in terms of polystyrene.

-   -   GPC instrument: HLC-8120 (manufactured by Tosoh Corporation)     -   Column: TSK gel Multipore HXL-M (manufactured by Tosoh         Corporation, 7.8 mmID (Inner Diameter)×30.0 cm)     -   Eluent: Tetrahydrofuran (THF)

At the time of forming the magnetic layer, it is possible to use a curing agent together with a resin usable as the aforementioned binder. In an aspect, the curing agent can be a thermosetting compound which is a compound experiencing a curing reaction (crosslinking reaction) by heating. In another aspect, the curing agent can be a photocurable compound experiencing a curing reaction (crosslinking reaction) by light irradiation. The curing agent experiences a curing reaction in the manufacturing process of the magnetic recording medium. In this way, at least a portion of the curing agent can be contained in the magnetic layer, in a state of reacting (crosslinked) with other components such as the binder. The curing agent is preferably a thermosetting compound which is suitably polyisocyanate. For details of polyisocyanate, paragraphs “0124” and “0125” in JP2011-216149A can be referred to. The curing agent can be used by being added to the composition for forming a magnetic layer, in an amount of 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binder and preferably in an amount of 50.0 to 80.0 parts by mass from the viewpoint of improving the hardness of the magnetic layer.

Abrasive

The magnetic layer of the magnetic recording medium contains an abrasive. The abrasive refers to non-magnetic powder having a Mohs hardness of higher than 8, and is preferably non-magnetic powder having a Mohs hardness of equal to or higher than 9. The abrasive may be powder of an inorganic substance (inorganic powder) or powder of an organic substance (organic powder), and is preferably inorganic powder. The abrasive is preferably inorganic powder having a Mohs hardness of higher than 8, and even more preferably inorganic powder having Mohs hardness of equal to or higher than 9. The maximum value of the Mohs hardness is 10 which is the Mohs hardness of diamond. Specific examples of the abrasive include powder of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), TiC, cerium oxide, zirconium oxide (ZrO₂), diamond, and the like. Among these, alumina powder is preferable. Regarding the alumina powder, paragraph “0021” in JP2013-229090A can also be referred to. As a parameter of the particle size of the abrasive, specific surface area can be used. The larger the specific surface area, the smaller the particle size. It is preferable to use an abrasive having a specific surface area (hereinafter, described as “BET specific surface area”) of equal to or greater than 14 m²/g, which is measured for primary particles by a Brunauer-Emmett-Teller (BET) method. From the viewpoint of dispersibility, it is preferable to use an abrasive having a BET specific surface area of equal to or less than 40 m²/g. The content of the abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder.

Fatty Acid Ester

The magnetic recording medium contains a fatty acid ester in the magnetic layer. The magnetic recording medium may contain only one kind of fatty acid ester or two or more kinds of fatty acid esters. Examples of the fatty acid ester include esters of lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, elaidic acid, and the like. Specific examples thereof include butyl myristate, butyl palmitate, butyl stearate, neopentyl glycol dioleate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyl oleate, isocetyl stearate, isotridecyl stearate, octyl stearate, isooctyl stearate, amyl stearate, butoxyethyl stearate, and the like.

The content of the fatty acid ester as the content in the composition for forming a magnetic layer is, for example, 0.1 to 10.0 parts by mass and preferably 1.0 to 7.0 parts by mass, with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder. In a case where two or more kinds of different fatty acid esters are used as the fatty acid ester, the content refers to the total content thereof. In the present invention and the present specification, unless otherwise specified, the definition of the content is true for the content of other components. Furthermore, in the present invention and the present specification, unless otherwise specified, regarding a certain component, one kind of the component may be used, or two or more kinds thereof may be used.

In a case where the magnetic recording medium has a non-magnetic layer between the non-magnetic support and the magnetic layer, the content of the fatty acid ester in the composition for forming a non-magnetic layer with respect to 100.0 parts by mass of non-magnetic powder is, for example, 0 to 10.0 parts by mass and preferably 0.1 to 8.0 parts by mass.

Other Components

Other Lubricants

The magnetic recording medium contains a fatty acid ester, which is one kind of lubricant, in at least the magnetic layer. A lubricant other than a fatty acid ester may be optionally contained in the magnetic layer and/or the non-magnetic layer. As described above, the lubricant contained in the non-magnetic layer can move to the magnetic layer and be present within the surface of the magnetic layer. Examples of the lubricant that can be optionally contained include a fatty acid. Furthermore, for example, a fatty acid amide or the like can be optionally contained. While the fatty acid ester is said to be a component that can function as a fluid lubricant, the fatty acid and the fatty acid amide are said to be components that can function as a boundary lubricant. The boundary lubricant is considered as a lubricant which can reduce the contact friction by being adsorbed onto the surface of powder (for example, ferromagnetic powder) and forming a strong lubricating film.

Examples of the fatty acid include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, elaidic acid, and the like. Among these, stearic acid, myristic acid, and palmitic acid are preferable, and stearic acid is more preferable. The fatty acid may be contained in the magnetic layer in the form of a salt such as a metal salt.

Examples of the fatty acid amide include amides of various fatty acids described above, such as lauric acid amide, myristic acid amide, palmitic acid amide, stearic acid amide, and the like.

Regarding the fatty acid and fatty acid derivatives (an amide, an ester, and the like), it is preferable that a fatty acid-derived portion of the fatty acid derivatives has a structure that is the same as or similar to the structure of the fatty acid used together with the derivatives. For example, in a case where stearic acid is used as a fatty acid, it is preferable to use a stearic acid ester and/or a stearic acid amide.

The content of the fatty acid in the composition for forming a magnetic layer with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder is 0 to 10.0 parts by mass for example, preferably 0.1 to 10.0 parts by mass, and more preferably 1.0 to 7.0 parts by mass. The content of the fatty acid amide in the composition for forming a magnetic layer with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder is 0 to 3.0 parts by mass for example, preferably 0 to 2.0 parts by mass, and more preferably 0 to 1.0 part by mass.

In a case where the aforementioned magnetic tape has a non-magnetic layer between the non-magnetic support and the magnetic layer, the content of the fatty acid in the composition for forming a non-magnetic layer with respect to 100.0 parts by mass of the non-magnetic powder is 0 to 10.0 parts by mass for example, preferably 1.0 to 10.0 parts by mass, and more preferably 1.0 to 7.0 parts by mass. The content of the fatty acid amide in the composition for forming a non-magnetic layer with respect to 100.0 parts by mass of the non-magnetic powder is 0 to 3.0 parts by mass for example, and preferably 0 to 1.0 part by mass.

Other Additives

The magnetic layer may further contain one or more kinds of additives if necessary. As one of the additives, the aforementioned curing agent and other lubricants can be exemplified. Examples of the additives that can be contained in the magnetic layer include non-magnetic powder, a lubricant, a dispersant, a dispersion aid, a fungicide, an antistatic agent, an antioxidant, and the like. As one of the additives which can be used in the magnetic layer containing the abrasive, the dispersant described in paragraphs “0012” to “0022” in JP2013-131285A can be exemplified as a dispersant for improving the dispersibility of the abrasive in the composition for forming a magnetic layer.

Examples of the dispersant also include known dispersants such as a carboxy group-containing compound and a nitrogen-containing compound. The nitrogen-containing compound may be any one of a primary amine represented by NH₂R, a secondary amine represented by NHR₂, and a tertiary amine represented by NR₃, for example. R represents any structure constituting the nitrogen-containing compound, and a plurality of R's present in the compound may be the same as or different from each other. The nitrogen-containing compound may be a compound (polymer) having a plurality of repeating structures in a molecule. The inventors of the present invention consider that because the nitrogen-containing portion of the nitrogen-containing compound functions as a portion adsorbed onto the surface of particles of the ferromagnetic hexagonal ferrite powder, the nitrogen-containing compound can act as a dispersant. Examples of the carboxy group-containing compound include fatty acids such as oleic acid. Regarding the carboxy group-containing compound, the inventors of the present invention consider that because the carboxy group functions as a portion adsorbed onto the surface of particles of the ferromagnetic hexagonal ferrite powder, the carboxy group-containing compound can act as a dispersant. It is also preferable to use the carboxy group-containing compound and the nitrogen-containing compound in combination.

Examples of the non-magnetic powder that can be contained in the magnetic layer include non-magnetic powder (hereinafter, described as “projection-forming agent” as well) which can contribute to the control of frictional characteristics by forming projections on the surface of the magnetic layer. As such a non-magnetic powder, it is possible to use various non-magnetic powders generally used in a magnetic layer. The non-magnetic powder may be inorganic powder or organic powder. In an aspect, from the viewpoint of uniformizing the frictional characteristics, it is preferable that the particle size distribution of the non-magnetic powder is not polydisperse distribution having a plurality of peaks in the distribution but monodisperse distribution showing a single peak. From the viewpoint of ease of availability of the monodisperse particles, the non-magnetic powder is preferably inorganic powder. Examples of the inorganic powder include powder of a metal oxide, a metal carbonate, a metal sulfate, a metal nitride, a metal carbide, a metal sulfide, and the like. The particles constituting the non-magnetic powder are preferably colloidal particles, and more preferably colloidal particles of an inorganic oxide. From the viewpoint of ease of availability of the monodisperse particles, the inorganic oxide constituting the colloidal particles of an inorganic oxide is preferably silicon dioxide (silica). The colloidal particles of an inorganic oxide are preferably colloidal silica (colloidal silica particles). In the present invention and the present specification, “colloidal particles” refer to the particles which can form a colloidal dispersion by being dispersed without being precipitated in a case where the particles are added in an amount of 1 g per 100 mL of at least one organic solvent among methyl ethyl ketone, cyclohexanone, toluene, ethyl acetate, and a mixed solvent containing two or more kinds of the solvents described above at any mixing ratio. In another aspect, the non-magnetic powder is also preferably carbon black. The mean particle size of the non-magnetic powder is 30 to 300 nm for example, and preferably 40 to 200 nm. The content of the non-magnetic powder in the magnetic layer is, with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder, preferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 parts by mass, because then the non-magnetic filler can demonstrate better the function thereof.

As various additives that can be optionally contained in the magnetic layer, commercially available products or those manufactured by known methods can be selected and used according to the desired properties.

The magnetic layer described so far can be provided on the surface of the non-magnetic support, directly or indirectly through a non-magnetic layer.

Non-Magnetic Layer

Next, a non-magnetic layer will be described.

The magnetic recording medium may have the magnetic layer directly on the surface of the non-magnetic support, or may have a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer. The non-magnetic powder contained in the non-magnetic layer may be inorganic powder or organic powder. Furthermore, carbon black or the like can also be used. Examples of the inorganic powder include powder of a metal, a metal oxide, a metal carbonate, a metal sulfate, a metal nitride, a metal carbide, a metal sulfide, and the like. These non-magnetic powders can be obtained as commercially available products, or can be manufactured by known methods. For details of the non-magnetic powder, paragraphs “0036” to “0039” in JP2010-24113A can be referred to. The content (filling rate) of the non-magnetic powder in the non-magnetic layer is preferably within a range of 50% to 90% by mass, and more preferably within a range of 60% to 90% by mass.

For other details of the binder, the additives, and the like of the non-magnetic layer, known techniques relating to the non-magnetic layer can be applied. For example, regarding the type and content of the binder, the type and content of the additives, and the like, known techniques relating to the magnetic layer can also be applied.

In the present invention and the present specification, the non-magnetic layer also includes a substantially non-magnetic layer which contains non-magnetic powder with a small amount of ferromagnetic powder as an impurity or by intention, for example. Herein, the substantially non-magnetic layer refers to a layer having a remnant flux density of equal to or lower than 10 mT or a coercive force of equal to or lower than 7.96 kA/m (100 Oe) or having a remnant flux density of equal to or lower than 10 mT and a coercive force of equal to or lower than 7.96 kA/m (100 Oe). It is preferable that the non-magnetic layer does not have remnant flux density and coercive force.

Non-Magnetic Support

Next, a non-magnetic support (hereinafter, simply described as “support” as well) will be described.

Examples of the non-magnetic support include known supports such as biaxially oriented polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide imide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable. These supports may be subjected to corona discharge, a plasma treatment, an easy adhesion treatment, a heat treatment, and the like in advance.

Back Coating Layer

The magnetic recording medium can have a back coating layer containing non-magnetic powder and a binder, on a surface side of the non-magnetic support opposite to a surface side provided with the magnetic layer. It is preferable that the back coating layer contains either or both of carbon black and inorganic powder. Regarding the binder contained in the back coating layer and various additives which can be optionally contained therein, known techniques relating to the back coating layer can be applied, and known techniques relating to the formulation of the magnetic layer and/or the non-magnetic layer can also be applied.

Various Thicknesses

The thickness of the non-magnetic support and each layer in the magnetic recording medium will be described below.

The thickness of the non-magnetic support is 3.0 to 80.0 μm for example, preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

The thickness of the magnetic layer can be optimized according to the saturation magnetization of the magnetic head to be used, the length of head gap, the band of recording signals, and the like. The thickness of the magnetic layer is generally 10 nm to 100 nm. From the viewpoint of high-density recording, the thickness of the magnetic layer is preferably 20 to 90 nm, and more preferably 30 to 70 nm. The magnetic layer may be constituted with at least one layer, and may be separated into two or more layers having different magnetic characteristics. Furthermore, the constitution relating to known multi-layered magnetic layers can be applied. In a case where the magnetic layer is separated into two or more layers, the thickness of the magnetic layer means the total thickness of the layers.

The thickness of the non-magnetic layer is equal to or greater than 50 nm for example, preferably equal to or greater than 70 nm, and more preferably equal to or greater than 100 nm. In contrast, the thickness of the non-magnetic layer is preferably equal to or less than 800 nm, and more preferably equal to or less than 500 nm.

The thickness of the back coating layer is preferably equal to or less than 0.9 μm, and more preferably 0.1 to 0.7 μm.

The thickness of each layer and the non-magnetic support of the magnetic recording medium can be measured by known film thickness measurement methods. For example, a cross section of the magnetic recording medium in a thickness direction is exposed by known means such as ion beams or a microtome, and then the exposed cross section is observed using a scanning electron microscope. By observing the cross section, a thickness of one site in the thickness direction or an arithmetic mean of thicknesses of two or more randomly extracted sites, for example, two sites can be determined as various thicknesses. Furthermore, as the thickness of each layer, a design thickness calculated from the manufacturing condition may be used.

Manufacturing Process

Preparation of Composition for Forming Each Layer

The step of preparing a composition for forming the magnetic layer, the non-magnetic layer, or the back coating layer generally includes at least a kneading step, a dispersion step, and a mixing step that is performed if necessary before and after the aforementioned steps. Each of the aforementioned steps may be divided into two or more stages. The components used for preparing the composition for forming each layer may be added at the initial stage or in the middle of any of the above steps. As a solvent, it is possible to use one kind of solvent or two or more kinds of solvents generally used for manufacturing a coating-type magnetic recording medium. Regarding the solvent, for example, paragraph “0153” in JP2011-216149A can be referred to. Furthermore, each of the components may be added in divided portions in two or more steps. For example, the binder may be added in divided portions in the kneading step, the dispersion step, and the mixing step performed after dispersion to adjust viscosity. In order to manufacture the aforementioned magnetic recording medium, the manufacturing techniques known in the related art can be used in various steps. In the kneading step, it is preferable to use an instrument having strong kneading force, such as an open kneader, a continuous kneader, a pressurized kneader, or an extruder. For details of the kneading treatment, JP1989-106338A (JP-H01-106338A) and JP1989-79274A (JP-H01-79274A) can be referred to. As a disperser, known ones can be used. The composition for forming each layer may be filtered by a known method before being subjected to a coating step. The filtration can be performed using a filter, for example. As the filter used for the filtration, for example, it is possible to use a filter having a pore size of 0.01 to 3 μm (for example, a filter made of glass fiber, a filter made of polypropylene, or the like).

Regarding the dispersion treatment for the composition for forming a magnetic layer, as described above, it is preferable to inhibit the occurrence of chipping. In order to inhibit chipping, in the step of preparing the composition for forming a magnetic layer, it is preferable to perform the dispersion treatment for the ferromagnetic hexagonal ferrite powder in two stages, such that coarse aggregates of the ferromagnetic hexagonal ferrite powder are disintegrated in the first stage of the dispersion treatment and then the second stage of the dispersion treatment is performed in which the collision energy applied to the particles of the ferromagnetic hexagonal ferrite powder due to the collision with dispersion beads is smaller than in the first dispersion treatment. According to the dispersion treatment described above, it is possible to achieve both of the improvement of dispersibility of the ferromagnetic hexagonal ferrite powder and the inhibition of occurrence of chipping.

Examples of preferred aspects of the aforementioned two-stage dispersion treatment include a dispersion treatment including a first stage of obtaining a dispersion liquid by performing a dispersion treatment on the ferromagnetic hexagonal ferrite powder, the binder, and the solvent in the presence of first dispersion beads, and a second stage of performing a dispersion treatment on the dispersion liquid obtained by the first stage in the presence of second dispersion beads having a bead size and a density smaller than a bead size and a density of the first dispersion beads. Hereinafter, the dispersion treatment of the aforementioned preferred aspect will be further described.

In order to improve the dispersibility of the ferromagnetic hexagonal ferrite powder, it is preferable that the first and second stages described above are performed as a dispersion treatment preceding the mixing of the ferromagnetic hexagonal ferrite powder with other powder components. For example, in a case where the magnetic layer containing the abrasive and the aforementioned non-magnetic powder is formed, it is preferable to perform the aforementioned first and second stages as a dispersion treatment for a liquid (magnetic liquid) containing the ferromagnetic hexagonal ferrite powder, the binder, the solvent, and additives optionally added, before the abrasive and the non-magnetic powder are mixed with the liquid.

The bead size of the second dispersion beads is preferably equal to or less than 1/100 and more preferably equal to or less than 1/500 of the bead size of the first dispersion beads. Furthermore, the bead size of the second dispersion beads can be, for example, equal to or greater than 1/10,000 of the bead size of the first dispersion beads, but is not limited to this range. For example, the bead size of the second dispersion beads is preferably within a range of 80 to 1,000 nm. In contrast, the bead size of the first dispersion beads can be within a range of 0.2 to 1.0 mm, for example.

In the present invention and the present specification, the bead size is a value measured by the same method as used for measuring the aforementioned mean particle size of powder.

The second stage described above is preferably performed under the condition in which the second dispersion beads are present in an amount equal to or greater than 10 times the amount of the ferromagnetic hexagonal ferrite powder, and more preferably performed under the condition in which the second dispersion beads are present in an amount that is 10 to 30 times the amount of the ferromagnetic hexagonal ferrite powder, based on mass.

The amount of the first dispersion beads in the first stage is preferably within the above range.

The second dispersion beads are beads having a density smaller than that of the first dispersion beads. “Density” is obtained by dividing mass (unit: g) of the dispersion beads by volume (unit: cm³) thereof. The density is measured by the Archimedean method. The density of the second dispersion beads is preferably equal to or lower than 3.7 g/cm³, and more preferably equal to or lower than 3.5 g/cm³. The density of the second dispersion beads may be equal to or higher than 2.0 g/cm³ for example, and may be lower than 2.0 g/cm³. In view of density, examples of the second dispersion beads preferably include diamond beads, silicon carbide beads, silicon nitride beads, and the like. In view of density and hardness, examples of the second dispersion beads preferably include diamond beads.

The first dispersion beads are preferably dispersion beads having a density of higher than 3.7 g/cm³, more preferably dispersion beads having a density of equal to or higher than 3.8 g/cm³, and even more preferably dispersion beads having a density of equal to or higher than 4.0 g/cm³. The density of the first dispersion beads may be equal to or lower than 7.0 g/cm³ for example, and may be higher than 7.0 g/cm³. As the first dispersion beads, zirconia beads, alumina beads, or the like are preferably used, and zirconia beads are more preferably used.

The dispersion time is not particularly limited and may be set according to the type of the disperser used and the like.

Coating Step

The magnetic layer can be formed by, for example, directly coating the non-magnetic support with the composition for forming a magnetic layer or by performing multilayer coating by sequentially or simultaneously coating the support with the composition for forming a non-magnetic layer. The back coating layer can be formed by coating a surface, which is opposite to a surface provided with the magnetic layer (or a surface on which the magnetic layer is additionally provided), of the non-magnetic support with the composition for forming a back coating layer. For details of coating for forming each layer, paragraph “0066” in JP2010-231843A can be referred to.

Other Steps

Regarding various other steps for manufacturing the magnetic tape, paragraphs “0067” to “0070” in JP2010-231843A can be referred to. For example, regarding the alignment treatment, it is possible to apply various known techniques including those described in paragraph “0067” in JP2010-231843A without any limitation. As described above, from the viewpoint of controlling the XRD intensity ratio, it is preferable to perform a vertical alignment treatment as the alignment treatment. Regarding the alignment treatment, the above description can also be referred to.

One Aspect of Preferred Manufacturing Method

As a preferred method for manufacturing the magnetic recording medium, a manufacturing method can be exemplified in which the magnetic layer is vibrated so as to improve the uniformity of the thickness of the liquid film of the fatty acid ester on the surface of the magnetic layer. It is considered that in a case where the magnetic layer is vibrated, the liquid film of the fatty acid ester on the surface of the magnetic layer flows, and hence the uniformity of the thickness of the liquid film is improved.

That is, the magnetic recording medium can be manufactured by a manufacturing method including forming a magnetic layer by coating the non-magnetic support with the composition for forming a magnetic layer, which contains ferromagnetic hexagonal ferrite powder, a binder, an abrasive, and a fatty acid ester, and drying the composition and vibrating the formed magnetic layer. This manufacturing method is the same as the general method for manufacturing a magnetic recording medium except that the magnetic layer is vibrated in the aforementioned manufacturing method, and details of this manufacturing method are as described above.

Means for vibrating the magnetic layer is not particularly limited. For example, in the non-magnetic support on which the magnetic layer is formed, a surface of the support opposite to the magnetic layer is brought into contact with a vibrating unit, and in this way, the magnetic layer can be vibrated. The non-magnetic support on which the magnetic layer is formed may be caused to run in a state of being brought into contact with the vibrating unit. The vibrating unit may include, for example, an ultrasonic vibrator on the inside thereof so as to vibrate an article coming into contact with the unit. The vibration caused in the magnetic layer can be adjusted by the vibration frequency of the ultrasonic vibrator, the intensity thereof, and/or the contact time with the vibrating unit. For example, the contact time can be adjusted by the running speed of the non-magnetic support, on which the magnetic layer is formed, being coming into contact with the vibrating unit. The aforementioned vibrating conditions are not particularly limited, and may be set such that the aforementioned full width at half maximum of the spacing distribution, particularly, the full width at half maximum FWHM_(before) of the spacing distribution before the heating in a vacuum can be controlled. In order to set the vibrating conditions, a preliminary experiment can be performed before the actual manufacturing, such that the conditions are optimized.

The full width at half maximum FWHM_(after) of the spacing distribution after the heating in a vacuum tends to be reduced by strengthening the dispersion conditions of the composition for forming a magnetic layer (for example, increasing the number of times of dispersion, increasing the dispersion time, and the like) and/or strengthening the filtration conditions (for example, using a filter having a small pore size as a filter used for filtration, increasing the number of times of filtration using a filter, and the like). Presumably, this is because in this way, the granular substances contained in the composition for forming a magnetic layer, particularly, the non-magnetic powder which can function as a projection-forming agent described above may experience the improvement of the dispersibility and/or the uniformity of the particle size, and hence the uniformity of the height of the projection present on the surface of the magnetic layer may be improved. The dispersion conditions and/or the filtration conditions can also be optimized by performing a preliminary experiment before the actual manufacturing.

Regarding the magnetic recording medium having a magnetic layer containing carbon black, in order to reduce the full width at half maximum FWHM_(after) of the spacing distribution after the heating in a vacuum, it is effective to use a dispersant, which is for improving the dispersibility of the carbon black, as a component of the magnetic layer. As one of the dispersants for carbon black, an organic tertiary amine can be exemplified. Regarding the organic tertiary amine, paragraphs “0011” to “0018” and “0021” in JP2013-049832A can be referred to. The organic tertiary amine is more preferably trialkylamine. The alkyl group that the trialkylamine has is preferably an alkyl group having 1 to 18 carbon atoms. The three alkyl groups that the trialkylamine has may be the same as or different from each other. For details of the alkyl group, paragraphs “0015” and “0016” in JP2013-049832A can be referred to. The trialkylamine is particularly preferably trioctylamine.

In the manner described so far, a magnetic recording medium according to an aspect of the present invention can be obtained. Here, the aforementioned manufacturing method is merely an example, and the value of each of the XRD intensity ratio, the squareness ratio in a vertical direction, FWHM_(before) and FWHM_(after) of the magnetic layer, and the difference (S_(after)−S_(before)) can be controlled within the aforementioned range by any means that can adjust the value, and this aspect is also included in the present invention.

The aforementioned magnetic recording medium according to an aspect of the present invention can be a tape-like magnetic recording medium (magnetic tape), for example. Generally, the magnetic tape is distributed and used in a state of being accommodated in a magnetic tape cartridge. In the magnetic tape, in order to enable head tracking servo to be performed in a drive, a servo pattern can also be formed by a known method. By mounting the magnetic tape cartridge on a drive (referred to as “magnetic tape device” as well) and running the magnetic tape in the drive such that a magnetic head contacts and slides on a surface of the magnetic tape (surface of a magnetic layer), information is recorded on the magnetic tape and reproduced. In order to continuously or intermittently perform repeated reproduction of the information recorded on the magnetic tape, the magnetic tape is caused to repeatedly run in the drive. According to an aspect of the present invention, it is possible to provide a magnetic tape in which the electromagnetic conversion characteristics thereof hardly deteriorate even if the head repeatedly slides on the surface of the magnetic layer while the tape is repeatedly running. Here, the magnetic recording medium according to an aspect of the present invention is not limited to the magnetic tape. The magnetic recording medium according to an aspect of the present invention is suitable as various magnetic recording media (a magnetic tape, a disc-like magnetic recording medium (magnetic disc), and the like) used in a sliding-type magnetic recording and/or reproduction device. The sliding-type device refers to a device in which a head contacts and slides on a surface of a magnetic layer in a case where information is recorded on a magnetic recording medium and/or the recorded information is reproduced. Such a device includes at least a magnetic tape and one or more magnetic heads for recording and/or reproducing information.

In the aforementioned sliding-type device, as the running speed of the magnetic tape is increased, it is possible to shorten the time taken for recording information and reproducing the recorded information. The running speed of the magnetic tape refers to a relative speed of the magnetic tape and the magnetic head. Generally, the running speed is set in a control portion of the device. As the running speed of the magnetic tape is increased, the pressure increases which is applied to both the surface of the magnetic layer and the magnetic head in a case where the surface of the magnetic layer and the magnetic head come into contact with each other. As a result, either or both of head scraping and magnetic layer scraping tend to easily occur. Accordingly, it is considered that the higher the running speed, the easier it is for the electromagnetic conversion characteristics to deteriorate during the repeated sliding. In the field of magnetic recording, the improvement of recording density is required. However, as the recording density is increased, the influence of the signal interference between the adjacent heads becomes stronger, and hence the electromagnetic conversion characteristics tend to be more easily deteriorate when the spacing loss is increased due to the repeated sliding. As described so far, as the running speed and the recording density are increased further, the deterioration of the electromagnetic conversion characteristics during the repeated sliding tends to be more apparent. In contrast, even in this case, according to the magnetic recording medium of an aspect of the present invention, it is possible to inhibit the deterioration of the electromagnetic conversion characteristics during the repeated sliding. The magnetic tape according to an aspect of the present invention is suitable for being used in a sliding-type device in which the running speed of the magnetic tape is, for example, equal to or higher than 5 m/sec (for example, 5 to 20 m/sec). In addition, the magnetic tape according to an aspect of the present invention is suitable as a magnetic tape for recording and reproducing information at a line recording density of equal to or higher than 250 kfci, for example. The unit kfci is the unit of a line recording density (this unit cannot be expressed in terms of the SI unit system). The line recording density can be equal to or higher than 250 kfci or equal to or higher than 300 kfci, for example. Furthermore, the line recording density can be equal to or lower than 800 kfci or higher than 800 kfci, for example.

EXAMPLES

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to the aspects shown in the examples. In the following description, unless otherwise specified, “part” and “%” represent “part by mass” and “% by mass” respectively. Furthermore, unless otherwise specified, the steps and the evaluations described below were performed in an environment with an atmospheric temperature of 23° C.±1° C.

Example 1

The formulations of compositions for forming each layer will be shown below.

Formulation of Composition for Forming Magnetic Layer

-   -   Magnetic liquid     -   Plate-like ferromagnetic hexagonal ferrite powder (M-type barium         ferrite): 100.0 parts     -   (activation volume: 1,500 nm³)     -   Oleic acid: 2.0 parts     -   Vinyl chloride copolymer (MR-104 manufactured by ZEON         CORPORATION): 10.0 parts     -   SO₃Na group-containing polyurethane resin: 4.0 parts     -   (weight-average molecular weight: 70,000, SO₃Na group: 0.07         meq/g)     -   Amine-based polymer (DISPERBYK-102 manufactured by BYK-Chemie         GmbH): 6.0 parts     -   Methyl ethyl ketone: 150.0 parts     -   Cyclohexanone: 150.0 parts     -   (Abrasive liquid)     -   α-Alumina: 6.0 parts     -   (BET specific surface area: 19 m²/g, Mohs hardness: 9)     -   SO₃Na group-containing polyurethane resin: 0.6 parts     -   (weight-average molecular weight: 70,000, SO₃Na group: 0.1         meq/g)     -   2,3-Dihydroxynaphthalene: 0.6 parts     -   Cyclohexanone: 23.0 parts     -   (Projection-forming agent liquid)     -   Colloidal silica: 2.0 parts         -   (mean particle size: 80 nm)     -   Methyl ethyl ketone: 8.0 parts     -   (Lubricant and curing agent liquid)     -   Stearic acid: 3.0 parts     -   Stearic acid amide: 0.3 parts     -   Butyl stearate: see Table 1     -   Methyl ethyl ketone: 110.0 parts     -   Cyclohexanone: 110.0 parts     -   Polyisocyanate (CORONATE (registered trademark) L manufactured         by Tosoh Corporation): 3.0 parts

Formulation of Composition for Forming Non-Magnetic Layer

-   -   Non-magnetic inorganic powder a iron oxide: 100.0 parts         -   (mean particle size: 10 nm, BET specific surface area: 75             m²/g)     -   Carbon black: 25.0 parts         -   (mean particle size: 20 nm)     -   Electron beam-curable vinyl chloride copolymer: 13.0 parts     -   Electron beam-curable polyurethane resin: 6.0 parts     -   Butyl stearate: see Table 1     -   Stearic acid: 1.0 part     -   Cyclohexanone: 300.0 parts     -   Methyl ethyl ketone: 300.0 parts

Formulation of Composition for Forming Back Coating Layer

-   -   Non-magnetic inorganic powder a iron oxide: 80.0 parts         -   (mean particle size: 0.15 μm, BET specific surface area: 52             m²/g)     -   Carbon black: 20.0 parts         -   (mean particle size: 20 nm)     -   Vinyl chloride copolymer: 13.0 parts     -   Sulfonate group-containing polyurethane resin: 6.0 parts     -   Phenyl phosphonate: 3.0 parts     -   Cyclohexanone: 155.0 parts     -   Methyl ethyl ketone: 155.0 parts     -   Stearic acid: 3.0 parts     -   Butyl stearate: 3.0 parts     -   Polyisocyanate: 5.0 parts     -   Cyclohexanone: 200.0 parts

Preparation of Composition for Forming Magnetic Layer

The composition for forming a magnetic layer was prepared by the following method.

The aforementioned various components of a magnetic liquid were dispersed for 24 hours by a batch-type vertical sand mill by using zirconia beads (first dispersion beads, density: 6.0 g/cm³) having a bead size of 0.5 mm (first stage) and then filtered using a filter having a pore size of 0.5 μm, thereby preparing a dispersion liquid A. The amount of the used zirconia beads was 10 times the mass of the ferromagnetic hexagonal ferrite powder based on mass.

Then, the dispersion liquid A was dispersed for the time shown in Table 1 by a batch-type vertical sand mill by using diamond beads (second dispersion beads, density: 3.5 g/cm³) having a bead size shown in Table 1 (second stage), and the diamond beads were separated using a centrifuge, thereby preparing a dispersion liquid (dispersion liquid B). The following magnetic liquid is the dispersion liquid B obtained in this way.

The aforementioned various components of an abrasive liquid were mixed together and put into a horizontal beads mill disperser together with zirconia beads having a bead size of 0.3 mm, and the volume thereof was adjusted such that bead volume/(volume of abrasive liquid)+bead volume equaled 80%. The mixture was subjected to a dispersion treatment by using the beads mill for 120 minutes, and the liquid formed after the treatment was taken out and subjected to ultrasonic dispersion and filtration treatment by using a flow-type ultrasonic dispersion and filtration device. In this way, an abrasive liquid was prepared.

The prepared magnetic liquid and abrasive liquid as well as the remaining components were introduced into a dissolver stirrer, stirred for 30 minutes at a circumferential speed of 10 m/sec, and then treated with a flow-type ultrasonic disperser at a flow rate of 7.5 kg/min in the pass number shown in Table 1. Thereafter, the resultant was filtered through a filter having a pore size shown in Table 1, thereby preparing a composition for forming a magnetic layer.

The activation volume of the ferromagnetic hexagonal ferrite powder described above is a value measured and calculated using the powder that was in the same powder lot as the ferromagnetic hexagonal ferrite powder used for preparing the composition for forming a magnetic layer. The activation volume was measured using a vibrating sample fluxmeter (manufactured by TOEI INDUSTRY, CO., LTD.) by setting a magnetic field sweep rate to be 3 minutes and 30 minutes for a coercive force Hc measurement portion, and calculated from the relational expression described above. The activation volume was measured in an environment with a temperature of 23° C.±1° C.

Preparation of Composition for Forming Non-Magnetic Layer

The aforementioned various components of a composition for forming a non-magnetic layer were dispersed by a batch-type vertical sand mill for 24 hours by using zirconia beads having a bead size of 0.1 mm and then filtered using a filter having a pore size of 0.5 μm, thereby preparing a composition for forming a non-magnetic layer.

Preparation of Composition for Forming Back Coating Layer

Among the aforementioned various components of a composition for forming a back coating layer, the components except for the lubricant (stearic acid and butyl stearate), polyisocyanate, and 200.0 parts of cyclohexanone were kneaded and diluted using an open kneader and then subjected to a dispersion treatment in 12 passes by a horizontal beads mill disperser by using zirconia beads having a bead size of 1 mm by setting a bead filling rate to be 80% by volume, a circumferential speed of the rotor tip to be 10 m/sec, and a retention time per pass to be 2 minutes. Then, other components described above were added thereto, followed by stirring with a dissolver. The obtained dispersion liquid was filtered using a filter having a pore size of 1 μm, thereby preparing a composition for forming a back coating layer.

Preparation of Magnetic Tape

A polyethylene naphthalate support having a thickness of 5.0 μm was coated with the composition for forming a non-magnetic layer such that the thickness thereof became 100 nm after drying, and the composition was dried. Then, the support was irradiated with electron beams with energy of 40 kGy at an acceleration voltage of 125 kV. The irradiated surface was then coated with the composition for forming a magnetic layer such that the thickness thereof became 70 nm after drying, thereby forming a coating layer. While the coating layer is remaining wet (undried state), in an alignment zone, a magnetic field having an intensity shown in Table 1 was applied thereto in a direction perpendicular to the surface of the coating layer so as to perform a vertical alignment treatment, and then the coating layer was dried.

Thereafter, the support on which the coating layer was formed was installed in the vibrating apparatus shown in FIG. 1 such that a surface, which was opposite to a surface on which the coating layer was formed, of the support came into contact with the vibrating unit, and the coating layer was vibrated by transporting the support (reference 101 in FIG. 1), on which the coating layer was formed, at a transport speed of 0.5 m/sec. In FIG. 1, reference 102 represents guide rollers (reference 102 is assigned to one of two guide rollers), reference 103 represents the vibrating apparatus (vibrating unit including an ultrasonic vibrator), and the arrow represents the transport direction. Table 1 shows the time from when any site in the support on which the coating layer was formed starts to come into contact with the vibrating unit to when the contact was finished (vibrating time). The used vibrating unit includes the ultrasonic vibrator on the inside thereof. The coating layer was vibrated by setting the vibration frequency and the intensity of the ultrasonic vibrator to be the values shown in Table 1.

Subsequently, a surface, which was opposite to the surface on which the non-magnetic layer and the magnetic layer were formed, of the support was coated with the composition for forming a back coating layer such that the thickness thereof became 0.40 μm after drying, and the composition was dried. Then, by using a calender rolls constituted solely with metal rolls, a surface smoothing treatment (calender treatment) was performed at a calender treatment speed of 80 m/min, a line pressure of 300 kg/cm (294 kN/m), and a calender temperature (calender roll surface temperature) of 90° C.

Subsequently, the resultant was subjected to a heat treatment for 36 hours in an environment with an atmospheric temperature of 70° C. After the heat treatment, the resultant was slit in a width of ½ inches (0.0127 meters), and by using a tape cleaning apparatus, which was obtained by mounting non-woven cloth and a razor blade on an apparatus including a device winding off and winding up the slit product such that the non-woven cloth and the razor blade are pressed on the surface of the magnetic layer, the surface of the magnetic layer was cleaned. Then, by using a commercial servowriter, a servo pattern was formed on the magnetic layer. In this way, a magnetic tape of Example 1 was obtained.

Evaluation of Deterioration of Electromagnetic Conversion Characteristics (Signal-to-Noise-Ratio; SNR)

The electromagnetic conversion characteristics of the magnetic tape of Example 1 were measured using a ½-inch (0.0127 meters) reel tester, to which a head was fixed, by the following method.

The running speed of the magnetic tape (relative speed of magnetic head/magnetic tape) was set to be the value shown in Table 1. A Metal-In-Gap (MIG) head (gap length: 0.15 μm, track width: 1.0 μm) was used as a recording head, and as a recording current, a recording current optimal for each magnetic tape was set. As a reproducing head, a Giant-Magnetoresistive (GMR) head having an element thickness of 15 nm, a shield gap of 0.1 μm, and a lead width of 0.5 μm was used. Signals were recorded at a line recording density shown in Table 1, and the reproduced signals were measured using a spectrum analyzer manufactured by ShibaSoku Co., Ltd. A ratio between an output value of carrier signals and integrated noise in the entire bandwidth of the spectrum was taken as SNR. For measuring SNR, the signals of a portion of the magnetic tape, in which signals were sufficiently stabilized after running, were used.

Under the above conditions, each magnetic tape was caused to perform reciprocating running in 5,000 passes at 1,000 m/1 pass in an environment with a temperature of 40° C. and a relative humidity of 80%, and then SNR was measured. Then, a difference between SNR of the 1^(st) pass and SNR of the 5,000^(th) pass (SNR of the 5,000^(th) pass-SNR of the 1^(st) pass) was calculated.

The recording and reproduction described above were performed by causing the head to slide on a surface of the magnetic layer of the magnetic tape.

Examples 2 to 23

Magnetic tapes were prepared in the same manner as in Example 1 except that various items shown in Table 1 were changed as shown in Table 1, and the deterioration of the electromagnetic conversion characteristics (SNR) of the prepared magnetic tapes was evaluated.

In Table 1, in the example for which “N/A” is described in the column of Dispersion beads and the column of Time, the composition for forming a magnetic layer was prepared without performing the second stage in the dispersion treatment for the magnetic liquid.

In Table 1, in the example for which “N/A” is described in the column of Magnetic field intensity for vertical alignment treatment, the magnetic layer was formed without performing the alignment treatment.

In Table 1, in the example for which “not performed” is described in the column of Staying time in cooling zone and in the column of Burnishing treatment before curing treatment, the magnetic tape was prepared by a manufacturing process in which a cooling zone is not included in the magnetic layer-forming step and the burnishing treatment as well as the wiping treatment were not performed before the curing treatment.

In Table 1, in the examples for which “N/A” is described in the column of Ultrasonic vibrating conditions, the magnetic tape was prepared by a manufacturing process in which vibration was not performed.

A portion of each of the prepared magnetic tapes was used for the evaluation of the deterioration of electromagnetic conversion characteristics (SNR), and the other portion thereof was used for physical property evaluation described below.

Evaluation of Physical Properties of Magnetic Tape

(1) XRD Intensity Ratio

From each of the magnetic tapes of Examples 1 to 23, tape samples were cut.

By using a thin film X-ray diffractometer (SmartLab manufactured by Rigaku Corporation), X-rays were caused to enter a surface of the magnetic layer of the cut tape sample, and In-Plane XRD was performed by the method described above.

From the X-ray diffraction spectrum obtained by In-Plane XRD, a peak intensity Int (114) of a diffraction peak of (114) plane and a peak intensity Int (110) of a diffraction peak of (110) plane of the hexagonal ferrite crystal structure were determined, and the XRD intensity ratio (Int (110)/Int (114)) was calculated.

(2) Squareness Ratio in Vertical Direction

For each of the magnetic tapes of Examples 1 to 23, by using a vibrating sample fluxmeter (manufactured by TOEI INDUSTRY, CO., LTD.), a squareness ratio in a vertical direction was determined by the method described above.

(3) Full Widths at Half Maximum FWHM_(before) and FWHM_(after) of Spacing Distribution Before and after Heating in Vacuum

By using Tape Spacing Analyzer (TSA, manufactured by MicroPhysics, Inc.), full widths at half maximum FWHM_(before) and FWHM_(after) of the spacing distribution before and after the heating in a vacuum were determined by the following method.

On the surface of the magnetic layer of the magnetic tape, a glass plate included in TSA was disposed. In this state, a hemisphere, which was included in TSA as a pressing member and made of urethane, was pressed on the surface of the back coating layer of the magnetic tape at a pressure of 5.05×10⁴ N/m (0.5 atm). In this state, a certain area (150,000 to 200,000 μm²) within the surface of the magnetic layer of the magnetic tape was irradiated with white light from a stroboscope included in TSA through the glass plate, and the obtained reflected light was received by a Charge-Coupled Device (CCD) through a dichroic filter (filter that selectively transmits light having a wavelength of 633 nm). In this way, an image of interference fringes caused in concavities and convexities in this area was obtained.

The image was divided into 300,000 points, and a distance (spacing) between the surface of the glass plate on the magnetic tape side and the surface of the magnetic layer of the magnetic tape was determined for each point and made into a histogram. The Gaussian distribution was fitted to the histogram, and the obtained full width at half maximum was taken as a full width at half maximum of the spacing distribution.

The magnetic tape was heated in a vacuum by being kept for 24 hours in a constant temperature vacuum drier with an internal atmospheric temperature of 70° C. to 90° C. and a degree of vacuum of equal to or higher than 200 Pa and equal to or lower than 0.01 MPa.

(4) Difference (S_(after)−S_(before))

From a mode of the histogram after the heating in a vacuum obtained in the aforementioned section (3), a mode of the histogram before the heating in a vacuum was subtracted, thereby calculating the difference (S_(after)−S_(before)).

The results are shown in Table 1 (Tables 1-1 and 1-2).

TABLE 1-1 Dispersion treatment for magnetic liquid Second stage Intensity of Content of butyl stearate Dispersion beads magnetic (part) Formulation amount field for Composition Composition (mass of beads with respect vertical for forming for forming to mass of ferromagnetic alignment magnetic non-magnetic Type Bead size hexagonal ferrite powder) Time treatment layer layer Example 1 Diamond 500 nm 10 times greater 1 h 0.15 T 1.0 4.0 Example 2 Diamond 500 nm 10 times greater 1 h 0.15 T 1.0 2.0 Example 3 Diamond 500 nm 10 times greater 1 h 0.20 T 1.0 4.0 Example 4 Diamond 500 nm 10 times greater 1 h 0.30 T 1.0 4.0 Example 5 Diamond 500 nm 10 times greater 1 h 0.50 T 1.0 4.0 Example 6 Diamond 500 nm 20 times greater 1 h 0.15 T 1.0 4.0 Example 7 Diamond 500 nm 10 times greater 1 h 0.30 T 1.0 4.0 Example 8 Diamond 500 nm 10 times greater 1 h 0.30 T 1.0 8.0 Example 9 Diamond 500 nm 10 times greater 1 h 0.30 T 1.0 4.0 Example 10 Diamond 500 nm 10 times greater 1 h 0.30 T 1.0 4.0 Example 11 Diamond 500 nm 10 times greater 1 h 0.30 T 1.0 4.0 Example 12 N/A N/A N/A N/A N/A 1.0 4.0 Example 13 N/A N/A N/A N/A N/A 1.0 4.0 Example 14 N/A N/A N/A N/A N/A 1.0 4.0 Example 15 Diamond 500 nm 10 times greater 1 h 0.15 T 1.0 4.0 Example 16 Diamond 500 nm 20 times greater 1 h 0.15 T 1.0 10.0 Example 17 Diamond 500 nm 20 times greater 1 h 0.15 T 0 0 Example 18 Diamond 500 nm 10 times greater 1 h 0.15 T 1.0 4.0 Example 19 N/A N/A N/A N/A N/A 1.0 4.0 Example 20 N/A N/A N/A N/A 0.15 T 1.0 4.0 Example 21 N/A N/A N/A N/A 0.30 T 1.0 4.0 Example 22 Diamond 500 nm 10 times greater 1 h 1.00 T 1.0 4.0 Example 23 Diamond 500 nm 10 times greater 1 h N/A 1.0 4.0 Conditions for preparing composition for forming magnetic layer Number Ultrasonic of times Number vibrating conditions of pass in of times Vibrating Vibration flow-type of time frequency Intensity ultrasonic filtration Pore size (sec) (kHz) (W) disperser (times) of filter Example 1 0.5 30 100 2 1 1.0 μm Example 2 0.5 30 100 2 1 1.0 μm Example 3 0.5 30 100 2 1 1.0 μm Example 4 0.5 30 100 2 1 1.0 μm Example 5 0.5 30 100 2 1 1.0 μm Example 6 0.5 30 100 2 1 1.0 μm Example 7 0.5 30 100 2 1 1.0 μm Example 8 0.5 30 100 2 1 1.0 μm Example 9 3 30 100 2 1 1.0 μm Example 10 0.5 30 100 30 5 0.5 μm Example 11 3 30 100 30 5 0.5 μm Example 12 N/A N/A N/A 2 1 1.0 μm Example 13 N/A N/A N/A 2 1 1.0 μm Example 14 N/A N/A N/A 2 1 1.0 μm Example 15 N/A N/A N/A 2 1 1.0 μm Example 16 0.5 30 100 2 1 1.0 μm Example 17 0.5 30 100 2 1 1.0 μm Example 18 0.5 30 100 1 1 1.0 μm Example 19 0.5 30 100 2 1 1.0 μm Example 20 0.5 30 100 2 1 1.0 μm Example 21 0.5 30 100 2 1 1.0 μm Example 22 0.5 30 100 2 1 1.0 μm Example 23 0.5 30 100 2 1 1.0 μm

TABLE 1-2 Magnetic layer Squareness ratio Running Line S_(after) − S_(before) FWHM_(before) FWHM_(after) XRD intensity ratio in vertical speed of recording SNR (nm) (nm) (nm) Int (110)/Int (114) direction magnetic tape density deterioration Example 1 3.9 6.7 6.6 0.5 0.70 6 m/s 270 kfci −0.5 dB Example 2 2.0 6.7 6.6 0.5 0.70 6 m/s 270 kfci −0.5 dB Example 3 4.0 6.6 6.7 1.5 0.75 6 m/s 270 kfci −0.9 dB Example 4 3.9 6.7 6.7 2.3 0.80 6 m/s 270 kfci −0.7 dB Example 5 4.0 6.6 6.8 4.0 0.85 6 m/s 270 kfci −0.5 dB Example 6 4.0 6.8 6.7 0.7 0.83 6 m/s 270 kfci −0.4 dB Example 7 4.1 6.7 6.8 2.3 0.80 8 m/s 300 kfci −0.8 dB Example 8 7.0 6.7 6.7 2.3 0.80 8 m/s 300 kfci −0.6 dB Example 9 4.0 4.0 6.8 2.3 0.80 8 m/s 300 kfci −0.6 dB Example 10 4.1 6.6 4.0 2.3 0.80 8 m/s 300 kfci −0.6 dB Example 11 4.0 4.0 4.0 2.3 0.80 8 m/s 300 kfci −0.3 dB Example 12 4.0 8.5 6.7 0.2 0.55 4 m/s 200 kfci −1.0 dB Example 13 4.0 8.5 6.7 0.2 0.55 6 m/s 270 kfci −3.0 dB Example 14 4.0 8.5 6.7 0.2 0.55 8 m/s 300 kfci −4.3 dB Example 15 4.0 8.5 6.8 0.5 0.70 6 m/s 270 kfci −2.5 dB Example 16 8.2 6.7 6.7 0.7 0.83 6 m/s 270 kfci −2.5 dB Example 17 0.0 6.7 6.7 0.7 0.83 6 m/s 270 kfci −2.5 dB Example 18 4.1 6.7 8.5 0.5 0.70 6 m/s 270 kfci −2.5 dB Example 19 4.1 6.8 6.7 0.2 0.55 6 m/s 270 kfci −2.3 dB Example 20 4.0 6.7 6.7 3.8 0.63 6 m/s 270 kfci −2.5 dB Example 21 3.9 6.8 6.8 5.0 0.75 6 m/s 270 kfci −2.5 dB Example 22 4.0 6.7 6.7 6.1 0.90 6 m/s 270 kfci −2.8 dB Example 23 4.0 6.6 6.6 0.3 0.66 6 m/s 270 kfci −2.5 dB

From the results shown in Table 1, it was confirmed that in Examples 1 to 11, in which each of the XRD intensity ratio, the squareness ratio in a vertical direction, FWHM_(before) and FWHM_(after) of the magnetic layer, and the difference (S_(after)−S_(before)) of the magnetic tape is within the range described above, the electromagnetic conversion characteristics hardly deteriorate even though reproduction is repeated by causing the head to slide on the surface of the magnetic layer, unlike in Examples 12 to 23.

In Examples 12 to 14, magnetic tapes of the same physical properties were used, but the magnetic tapes had different running speeds and different line recording densities. Through the comparison between Examples 12 to 14, it is possible to confirm that as the running speed or the line recording density of the magnetic tape is increased, the deterioration of the electromagnetic conversion characteristics during the repeated sliding becomes more apparent. In Examples 1 to 11, the deterioration of the electromagnetic conversion characteristics during the repeated sliding could be inhibited.

One aspect of the present invention can be useful in the technical field of magnetic recording media for data storage such as data backup tapes. 

What is claimed is:
 1. A magnetic recording medium comprising: a non-magnetic support; and a magnetic layer which is provided on the support and contains ferromagnetic powder and a binder, wherein the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive and a fatty acid ester, an intensity ratio (Int (110)/Int (114)) of a peak intensity Int (110) of a diffraction peak of (110) plane of a crystal structure of the hexagonal ferrite, determined by performing X-ray diffraction analysis on the magnetic layer by using an In-Plane method, to a peak intensity Int (114) of a diffraction peak of (114) plane of the crystal structure is equal to or higher than 0.5 and equal to or lower than 4.0, a squareness ratio of the magnetic recording medium in a vertical direction is equal to or higher than 0.65 and equal to or lower than 1.00, a full width at half maximum of a spacing distribution measured within a surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum is greater than 0 nm and equal to or smaller than 7.0 nm, a full width at half maximum of a spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum is greater than 0 nm and equal to or smaller than 7.0 nm, and a difference (S_(after)−S_(before)) between a spacing S_(after), which is measured within a surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum, and a spacing S_(before), which is measured within the surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum, is greater than 0 nm and equal to or smaller than 8.0 nm.
 2. The magnetic recording medium according to claim 1, wherein the squareness ratio in a vertical direction is equal to or higher than 0.65 and equal to or lower than 0.90.
 3. The magnetic recording medium according to claim 1, wherein the full width at half maximum of a spacing distribution measured within the surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum is equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.
 4. The magnetic recording medium according to claim 2, wherein the full width at half maximum of a spacing distribution measured within the surface of the magnetic layer by an optical interference method before the magnetic recording medium is heated in a vacuum is equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.
 5. The magnetic recording medium according to claim 1, wherein the full width at half maximum of the spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum is equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.
 6. The magnetic recording medium according to claim 2, wherein the full width at half maximum of the spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum is equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.
 7. The magnetic recording medium according to claim 3, wherein the full width at half maximum of the spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum is equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.
 8. The magnetic recording medium according to claim 4, wherein the full width at half maximum of the spacing distribution measured within the surface of the magnetic layer by an optical interference method after the magnetic recording medium is heated in a vacuum is equal to or greater than 3.0 nm and equal to or smaller than 7.0 nm.
 9. The magnetic recording medium according to claim 1, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 10. The magnetic recording medium according to claim 2, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 11. The magnetic recording medium according to claim 3, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 12. The magnetic recording medium according to claim 4, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 13. The magnetic recording medium according to claim 5, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 14. The magnetic recording medium according to claim 6, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 15. The magnetic recording medium according to claim 7, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 16. The magnetic recording medium according to claim 8, wherein the difference S_(after)−S_(before) is equal to or greater than 2.0 nm and equal to or smaller than 8.0 nm.
 17. The magnetic recording medium according to claim 1, further comprising: a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer.
 18. The magnetic recording medium according to claim 1, further comprising: a back coating layer containing non-magnetic powder and a binder on a surface, which is opposite to a surface provided with the magnetic layer, of the non-magnetic support.
 19. The magnetic recording medium according to claim 1, which is a magnetic tape. 